Creating or switching to a new tab feels slow

Confirmed on latest rc-proposed today on M10.

Here's a video showing it.

https://youtu.be/FcarlJiZtNg

On my beefy laptop, when opening a new tab, the UrlGridView for the list of top sites in the new tab view takes ~250ms to instantiate, of which almost half the time (115ms) is for instantiating the Loader that contains one Favicon.

After moving away the favicons cache folder, instantiating the favicons is almost instantaneous (they are being re-downloaded on a different thread), and instantiating the UrlGridView is down to 170ms, which is still expensive, but better.

I instrumented the favicon fetcher code, and the culprit seems to be the first icon being downloaded (usually because the favicon doesn’t actually exist, but we’re not tracking that, so we try to download it anyway because it hasn’t been cached on disk). Specifically, on my laptop instantiating the first QNetworkAccessManager takes ~100ms. One QNAM is being instantiated for each favicon that we fetch from the network, but the subsequent instantiations are free, only the first one is expensive. This would need some further investigation in Qt itself.

One optimization we could try to apply is to cache an empty file on disk for non-existent favicons, in order to avoid trying to re-download them over and over again.

I experimented with a very simple test application to measure how expensive it is to create instances of QNetworkAccessManager, and the results are consistent (and surprisingly similar on my beefy laptop and on an MX4):

int main(int argc, char** argv) {
  QCoreApplication app(argc, argv);
  for (int i = 0; i < 10; ++i) {
    QElapsedTimer timer;
    timer.start();
    QNetworkAccessManager nam;
    qDebug() << i << "elapsed:" << timer.elapsed();
  }
  return 0;
}

The first instance takes anywhere between 75 and 125ms to create, subsequent instances are free.

I finally got around to doing some profiling on actual touch devices, and the expensive operation was instantiating images synchronously for grid delegates. Doing it asynchronously speeds up the grid view creation significantly.

This bug was fixed in the package webbrowser-app - 0.23+16.10.20160615-0ubuntu1

---------------
webbrowser-app (0.23+16.10.20160615-0ubuntu1) yakkety; urgency=medium

  * Fix the anchoring of the contents in the bookmarks views. (LP:
    #1584385)
  * Various favicon optimizations. (LP: #1585285)
  * Load top site previews asynchronously to speed up the instantiation
    of the grid view. (LP: #1585285)
  * Fix cross-compilation for armhf on a x86* host. (LP: #1588788)
  * Update UA overrides (mobile) for youtube and gmail.

 -- Olivier Tilloy <email address hidden> Wed, 15 Jun 2016 09:35:09 +0000

Status changed to 'Confirmed' because the bug affects multiple users.